Process for manufacturing articles comprising polylactic acid polymers having improved heat resistance

Abstract

A process for thermoforming a discrete section of an amorphous pliable thermoplastic sheet in a temperature-controlled thermoforming mold to provide a discrete thermoformed section having one or more discrete shaped articles formed therein and to heat the shaped articles to a temperature below the T m but above the T g of the polylactic acid polymer in the sheet to induce crystallization of the polylactic acid polymer in the shaped articles. While unconstrained by the thermoforming mold, the discrete thermoformed section is tempered at a temperature such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of at least about 5% (as measured by X-ray diffraction) to thereby increase heat resistance of the shaped articles.

What is claimed is: 
     
         1 . A process for increasing the heat resistance of shaped thermoformed articles made from a thermoplastic sheet, comprising the following steps:
 (a) providing an amorphous pliable thermoplastic sheet comprising at least about 60% by weight of the amorphous pliable thermoplastic sheet of a polylactic acid polymer;   (b) thermoforming a discrete section of the amorphous pliable thermoplastic sheet in a temperature-controlled thermoforming mold to provide a discrete thermoformed section having one or more shaped articles formed therein and to impart to the shaped articles a temperature below the T m  but above the T g  of the polylactic acid polymer to thereby induce crystallization of the polylactic acid polymer in the shaped articles; and   (c) while unconstrained by the thermoforming mold, tempering the thermoformed section at a temperature above the T g  of the polylactic acid polymer such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of at least about 5% (as measured by X-ray diffraction) to thereby increase the heat resistance of the shaped articles.   
     
     
         2 . The process of  claim 1 , wherein the polylactic acid polymer comprises at least about 70% by weight L-polylactic acid polymer or D-polylactic acid polymer. 
     
     
         3 . The process of  claim 2 , wherein the polylactic acid polymer comprises at least about 85% by weight L-polylactic acid polymer or D-polylactic acid polymer. 
     
     
         4 . The process of  claim 2 , wherein the polylactic acid polymer comprises from about 72 to about 99% by weight L-polylactic acid polymer or D-polylactic acid polymer. 
     
     
         5 . The process of  claim 1 , which comprises the further step (d) of extruding the thermoplastic sheet prior to step (a) and wherein the extruded thermoplastic sheet comprises a crystallization inducing amount of at least one polylactic acid polymer nucleating agent. 
     
     
         6 . The process of  claim 1 , wherein step (a) is carried out by heating the thermoplastic sheet to a temperature at or above the T m  of the polylactic acid polymer. 
     
     
         7 . The process of  claim 6 , wherein step (a) is carried out by heating the thermoplastic sheet to a temperature of at least about 250° F. 
     
     
         8 . The process of  claim 7 , wherein step (a) is carried out by heating the thermoplastic sheet to a temperature of from about 300° to about 450° F. 
     
     
         9 . The process of  claim 1 , wherein step (b) is carried out by incrementally advancing the discrete section of the amorphous pliable thermoplastic sheet into the thermoforming mold to impart to the shaped articles in the thermoformed section a temperature of from about 100° to about 200° F. for a period of from about 0.2 to about 5 seconds. 
     
     
         10 . The process of  claim 9 , wherein the temperature imparted to the shaped articles in the thermoformed section in the thermoforming mold during step (b) is from about 140° to about 160° F. for a period of from about 0.5 to about 1.5 seconds. 
     
     
         11 . The process of  claim 1 , wherein a plurality of shaped articles are formed in the thermoformed section during step (b). 
     
     
         12 . The process of  claim 11  which comprises the further step (e) of detaching the shaped articles from the thermoformed section. 
     
     
         13 . The process of  claim 12 , wherein step (c) is carried out by cutting out the shaped articles from the thermoformed section. 
     
     
         14 . The process of  claim 11 , wherein the shaped articles are beverage cup lids. 
     
     
         15 . The process of  claim 1 , wherein step (c) is carried out by convection cooling of the discrete thermoformed section. 
     
     
         16 . The process of  claim 1 , wherein steps (b) and (c) are repeated sequentially at least once. 
     
     
         17 . The process of  claim 16 , wherein steps (b) and (c) are repeated sequentially at a rate of from about 5 to about 30 cycles per minute. 
     
     
         18 . The process of  claim 17 , wherein steps (b) and (c) are repeated sequentially at a rate of from about 20 to about 26 cycles per minute. 
     
     
         19 . The process of  claim 1 , wherein step (c) is carried out such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of from about 5 to about 45% (as measured by X-ray diffraction). 
     
     
         20 . The process of  claim 1 , wherein step (c) is carried out such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of at least about 8% (as measured by X-ray diffraction). 
     
     
         21 . The process of  claim 20 , wherein step (c) is carried out such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of from about 8 to about 20% (as measured by X-ray diffraction). 
     
     
         22 . The process of  claim 1 , wherein the thermoplastic sheet of step (a) comprises at least about 80% by weight polylactic acid polymer and from about 5 to about 20% by weight of a polylactic acid polymer nucleating agent. 
     
     
         23 . The process of  claim 22 , wherein the thermoplastic sheet of step (a) comprises at least about 85% by weight polylactic acid polymer and from about 12 to about 15% by weight of a polylactic acid polymer nucleating agent. 
     
     
         24 . The process of  claim 22 , wherein the polylactic acid polymer nucleating agent comprises talc. 
     
     
         25 . The process of  claim 24 , wherein the polylactic acid polymer nucleating agent comprises talc and Sukano na S516 in a weight ratio of talc:Sukano na S516 of from about 1:2 to about 4.5:1. 
     
     
         26 . The process of  claim 1 , wherein the tempering in step (c) is carried out for from about 10 to about 30 seconds. 
     
     
         27 . The process of  claim 26 , wherein the tempering in step (c) is carried out for from about 15 to about 20 seconds. 
     
     
         28 . The process of  claim 1 , wherein the amorphous pliable thermoplastic sheet of step (a) is formed by combining a first master compound comprising polylactic acid polymer and an inorganic polylactic acid polymer nucleating agent and a second first master compound comprising polylactic acid polymer and an organic polylactic acid polymer nucleating agent. 
     
     
         29 . A process for manufacturing shaped thermoformed articles having improved heat resistance from a thermoplastic sheet, comprising the following steps:
 (a) continuously extruding a thermoplastic sheet comprising a least about 60% by weight of the thermoplastic sheet of a polylactic acid polymer and a crystallization inducing amount of at least one polylactic acid polymer nucleating agent;   (b) heating the thermoplastic sheet of step (a) to a temperature at or above the T m  of the polylactic acid polymer to provide an amorphous pliable thermoplastic sheet;   (c) incrementally advancing a discrete section of the amorphous pliable thermoplastic sheet into a temperature-controlled thermoforming mold to provide a discrete thermoformed section having a plurality of shaped articles formed therein and to impart to the shaped articles a temperature below the T m  but above the T g  of the polylactic acid polymer to thereby induce crystallization of the polylactic acid polymer in the shaped articles;   (d) while unconstrained by the thermoforming mold, convection cooling the thermoformed section at a temperature above the T g  of the polylactic acid polymer such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of at least about 5% (as measured by X-ray diffraction) to thereby increase the heat resistance of the shaped articles; and   (e) after step (d), detaching each of the shaped articles from the thermoformed section;   wherein steps (b) through (e) are performed sequentially a plurality of times.   
     
     
         30 . The process of  claim 29 , wherein the polylactic acid polymer comprises at least about 70% by weight L-polylactic acid polymer or D-polylactic acid polymer. 
     
     
         31 . The process of  claim 30 , wherein the polylactic acid polymer comprises at least about 85% by weight L-polylactic acid polymer or D-polylactic acid polymer. 
     
     
         32 . The process of  claim 30 , wherein the polylactic acid polymer comprises from about 72 to about 99% by weight L-polylactic acid polymer or D-polylactic acid polymer. 
     
     
         33 . The process of  claim 29 , wherein step (b) is carried out by heating the thermoplastic sheet to a temperature of at least about 250° F. 
     
     
         34 . The process of  claim 33 , wherein step (b) is carried out by heating the thermoplastic sheet to a temperature of from about 300° to about 450° F. 
     
     
         35 . The process of  claim 29 , wherein the shaped articles in the thermoformed section are heated in the thermoforming mold during step (c) to a temperature of from about 100° to about 200° F. for a period of from about 0.2 to about 5 seconds. 
     
     
         36 . The process of  claim 35 , wherein the temperature imparted to the shaped articles in the thermoformed section in the thermoforming mold during step (c) is from about 140° to about 160° F. for a period of from about 0.5 to about 1.5 seconds. 
     
     
         37 . The process of  claim 29 , wherein step (e) is carried out by cutting out the shaped articles from the thermoformed section. 
     
     
         38 . The process of  claim 37 , wherein step (e) generates a residual thermoformed section after cutting out the shaped articles, and which comprises the further step (f) of recycling the residual thermoformed section when continuously extruding a thermoplastic sheet during step (a). 
     
     
         39 . The process of  claim 29 , wherein steps (b) through (d) are repeated sequentially at a rate of from about 5 to about 30 cycles per minute. 
     
     
         40 . The process of  claim 39 , wherein steps (b) through (d) are repeated sequentially at a rate of from about 20 to about 26 cycles per minute. 
     
     
         41 . The process of  claim 29 , wherein step (d) is carried out such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of from about 5 to about 45% (as measured by X-ray diffraction). 
     
     
         42 . The process of  claim 29 , wherein step (d) is carried out such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of at least about 8% (as measured by X-ray diffraction). 
     
     
         43 . The process of  claim 42 , wherein step (d) is carried out such that the polylactic acid polymer in the shaped articles achieves a degree of crystallization of from about 8 to about 20% (as measured by X-ray diffraction). 
     
     
         44 . The process of  claim 29 , wherein the thermoplastic sheet of step (a) comprises at least about 80% by weight polylactic acid polymer and from about 5 to about 20% by weight of a polylactic acid polymer nucleating agent. 
     
     
         45 . The process of  claim 44 , wherein the thermoplastic sheet of step (a) comprises at least about 85% by weight polylactic acid polymer and from about 12 to about 15% by weight of a polylactic acid polymer nucleating agent. 
     
     
         46 . The process of  claim 45 , wherein the polylactic acid polymer nucleating agent comprises talc. 
     
     
         47 . The process of  claim 46 , wherein the polylactic acid polymer nucleating agent comprises talc and Sukano na S516 in a weight ratio of talc:Sukano na S516 of from about 1:2 to about 4.5:1. 
     
     
         48 . The process of  claim 29 , wherein the convection cooling in step (d) is carried out for from about 10 to about 30 seconds. 
     
     
         49 . The process of  claim 48 , wherein the convection cooling in step (d) is carried out for from about 15 to about 20 seconds. 
     
     
         50 . The process of  claim 29 , wherein the thermoplastic sheet of step (a) is formed by continuously extruding a blend of a first master compound comprising polylactic acid polymer and an inorganic polylactic acid polymer nucleating agent and a second first master compound comprising polylactic acid polymer and an organic polylactic acid polymer nucleating agent.